Timing apparatus for high speed printer

ABSTRACT

A high speed printer wherein print characters are disposed in rows about the periphery of a rotating drum, wherein the particular character opposite a print line along the direction of rotation of the drum is denoted by the state of a counter which is advanced as each character row passes the print line, and wherein the counter is reset when a predetermined point on the drum passes said print line.

United States Patent Castoldi 51 Dec. 26, 1972 s41 TIMING APPARATUS FORHIGH 3,001,469 9/1961 Davies et a1. ....101/93 0 SPEED PRINTER 3,024,7233/1962 Wasserman ....101/93 C 3,117,514 1 1964 D ....l0l/93C 1 Invent:Fabml" Castoldl, Mllan, Italy 3,158,090 11/1964 ..101/93 c 3,220,34311/1965 Wasserman ..10l/93 C [73] Ass'gnee S2122: 3$? s g g: 3,314,3604/1967 Foster ..101/93 c x y 3,420,166 1/1969 Ellis ct a1. ..l0ll93 Cluso (T urin), Italy [22] Filed: June 1, 1970 Primary Examiner-William13. Penn Attorne Geor e V. Elt roth, Lewis P. Elbin er and [21] Appl'Frank Neuhaiser g g 52' us. c1. ..101/93 (2 ABSTRACT [51] Int. Cl. 1734A hi h d g spec printer wherein prmt characters are [58] Filed of Search..101/93 C, 340/149, disposed in rows aboutmtheggfiphml of imitating1.724 drum, wherein the particular character opposite a print line alongthe direction of rotation of the drum is [56] References cued denoted bythe state of a counter which is advanced as UNITED STATES PATENTS eachcharacter row passes the print line, and wherein 1 the counter is resetwhen a predetermined point on 2,757,605 8/1956 Dumey ..10l/93 C the drumpasses said print line. 2,776,618 1/1957 Hartley... .101/93 C 2,906,2009/1959 Pfleger ..101/93 C 7 Claims, 8 Drawing Figures COMPARISON ANDCONTROL ClRCUlT PATENTEDnaczs I972 SHEET 1 BF 3 CONTROL CIRCUiT LCOMPARISON AND CONTROL ClRCUlT.

Fabrl zl o CASTOLD/ INVENTOR.

ATTORNEY.

PATGNTED [15925 I973 3. 707 121 SHEET 3 [IF 3 w 33/ A 1 A 19 g L-' L, L21 LJ 22," 25 B zzJ 5 COMPARISON COMPARISON AND CONTROL u AND CONTROL 23CIRCUIT 23 c Rcu m7 Fig. 5 Fig.7

"a Fig. 8 x 36 u;::%x

Fig.6 g

COMPARISON 1 AND CONTROL Fabr'm 23 CIRCUIT M17 CASTOLDI INVEXTOR.

ATTORNEY.

TIMING APPARATUS FOR HIGH SPEED PRINTER BACKGROUND OF THE INVENTION Thepresent invention relates to high speed printers for use with digitaldata processing equipment and more particularly to timing devices andcircuits for the same.

It is known, in the field of peripheral output devices for electroniccomputers, that high printing speeds may be obtained with serial, orpreferably, parallel printers, by the principle known as printingon-the-fly. According to such principle, a type-carrying member isgenerally formed by a cylindrical ring on which the characters to beprinted are provided in relief and arranged circumferentially, or of adrum on which'the characters are repeated on a number of parallel rings.Such a typecarrying member is caused to rotate at a constant speed insuch a manner that all characters carried by said member pass a fixedgeneratrix of the drum or of the ring in a predetermined time interval.

Opposite to this fixed generatrix and in close proximity to the surfaceof the type-carrying member, a number of printing members, such ashammers, are arranged at fixed print positions, or, alternatively, oneor more hammers may be moved along the generatrix in order to occupysuccessively different print positions.

An inking ribbon and a print receiving member, usually a paper sheet,are interposed between the typecarrying member and the printing members.When, a character required to be printed at a definite print positionreaches, while rotating, the fixed generatrix and, therefore, isadjacent an associated hammer, the hammer is driven forward to press theprint receiving member and the inking ribbon against such character,under the action of an electromagnet or similar device. Accordingly, theshape of the character is transferred to the print receiving member.

For such types of on-the-fly printers, and also for non-impact printersemploying electrostatic, optical, and other techniques for transferringcharacter images to print receiving members by on-the-fiy techniques, itis necessary that it be known when a predetermined character is in theproper position for printing, in order to be able to control the drivingof one or more printing members at the proper moment.

Generally, the identification of a character and, accordingly, timingthe driving of the printing members, is provided by using a code disc,secured to the typecarrying member and rotating therewith. This codedischas reference indicia and timing means easily identifiable and radiallyarranged to correspond to the angular position of each character, or, ifnecessary, angularly displaced by a suitable angle relative to theangular positions of the characters.

For instance, if photoelectric cells are used for reading, or sensing,the timing and reference indicia, the code disc is opaque and theindicia are formed by arrangements of transparent windows through whichthe photo cells are suitably excited by a light source, therebygenerating electrical pulses whose combinations correspond to the binarycode combinations representing the different characters. The successionof these binary code combinations, which change continuously asdifferent characters approach the printing position, is compared withthe code combination representative of the character to be printed ateach print position. When both combinations coincide for a particularprint position, the printing member at such position is driven to carryout the print operation.

A simpler arrangement consists of generating a succession of pulses bymeans of indicia sensed by photoelectric cells or magnetic transducers,pick-ups, or equivalent means, each one of said pulses corresponding toa character position. These pulses advance a binary counter, whereby thecode combinations appearing on the counters parallel outputs representthe characters approaching the print generatrix. In this case areference signal must be generated at the beginning of each revolutionto reset the counter to its initial state. For this purpose, anadditional mark is used, which can be sensed by a second transducer, orby the same transducer used for the character pulses if properdiscriminating circuits are provided. This arrangement is simple andeconomical, but has some drawbacks which limit, or even prevent itsapplication in certain instances.

For example, if a magnetic transducer is used it must be associated-witha ferromagnetic wheel having teeth or notches, each tooth or notchcorresponding to a character, and having an additional tooth or notch todenote the beginning of each revolution. This additional tooth or notchis necessarily interposed between two adjacent character teeth ornotches, thereby preventing reduction below a. definite limit of thedimensions and, therefore, the inertia of the toothed wheel. Moreover,this additional tooth or notch disturbs the operation of the transducer,so that in correspondence with this tooth and the adjacent ones, itdelivers electrical signals of considerable lower level, appreciablydistorted, and having a frequency double the normal frequency. Thus,transducers of high sensitivity and having a greater frequency responseare required.

Therefore, it is the object of this invention to provide an improvedtiming device for on-the-fly printers.

SUMMARY OF THE INVENTION The disadvantages and limitations of the priorart are overcome by the timing device of the instant invention, whereinthe signalling of the beginning of every revolution is obtained by thedetection of the absence of at least one tooth, or more generally, ofone of the indicia in the normal succession of indicia carried by thetiming disc.

The illustrative description following hereinafter, in order to moreclearly explain the operation of the device and of the associatedcircuits, represents a preferred embodiment specifically employed in ahighspeed drum printer. However, it is evident that the invention may beused in all instances wherein the angular position of bodies rotating atsubstantially constant angular speeds, or wherein the position reachedby members repetitively carried along a closed path, must be identified;as, for example, in high-speed, on-the-fly printers of the serial typeand in chain printers, ribbon printers, or the like.

BRIEF DESCRIPTION OF THE DRAWING The invention will be described withreference to the accompanying drawing, wherein:

FIG. 1 is a block diagram of the preferred embodiment of the invention;

circuit of FIG. 1;

FIG. 6 shows in detail the arrangement of the characters on thetype-carrying member, as required by the variation of FIG. 5.

FIG. 7 is a block diagram of a second variation of the circuit of FIG.1; and

FIG. 8 is a block diagram of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT N In FIG. la type-carrying drum1 is caused to rotate at a constant speed by amotor 2. A ferromagnetictiming disc 3 is secured to the shaft of drum 1 and rotates therewith.On the periphery of disc 3 are suitable reference notches 4, angularlyspaced apart by the same angular pitch as the characters, FIG. 3, on thesurface of drum 1.

Opposite to a fixed generatrix of the drum, and in close proximity tothis generatrix, a suitable number of electromagnetically actuatedhammers is provided. For simplicity, only one of these hammers isrepresented in the figure, indicated by the reference numeral 7. Hammer7 is actuated by means of an electromagnet 8, controlled, in turn, by acontrol circuit 9. A print receiving member 5 and an inking ribbon 6 areinterposed between the type-carrying drum and the hammer set.

An electromagnetic transducer, or pick-up, 10 is arranged proximate tothe periphery of timing disc 3 at an appropriate angular position, anddelivers on its output 11 avoltage pulse each time a notch 4 of the disc3 passes by its reading, or sensing, head 12. The waveform (a), FIG. 4,of the pulses on output 11 constitutes a succession of approximatelysinusoidal waves of normal period P. The pulses delivered by pick-up 10are applied to the input of a square-pulse forming amplifier 13, whichgenerates substantially square pulses with a normal repetition period Pof constant amplitude and duration, waveform (b).

These square pulses are applied to input 14 of a oneshot 15, whichresponds to the negative trailing edge of each incoming pulse togenerate a positive square pulse having a well-defined duration D,substantially shorter than repetition period P, waveform (c).

The output pulses of one-shot 15 are applied to input 16 of a comparisonand control circuit 17, which also receives on input 23 code signalcombinations representing the characters to be printed. The pulses oninput 16 advance a binary counter, which thereby delivers code signalcombinations corresponding to the characters arranged in succession onrings of the typecarrying drum. These code signal combinations arecompared with the code signal combination of the character to beprinted. When both code signal combinations are the same, a pulse isdelivered on output 18, such pulse actuating electromagnet 8 of thehammer 7 associated with the print position at which such character mustbe printed. The instant at which this actuating pulse is transmitteddepends solely on the timing of the pulses applied to input 16.

The arrangement thus far described is well known in the art.

The output pulses of one-shot 15 are termed character pulses. Suchpulses are applied not only to comparison and control circuit 17, butalso to the input of a rapid-recovery one-shot 19, which deliverspulseshaving awell-defined duration T. Such a circuit, known in the art, ischaracterized in that if the control pulses applied to its input followeach other by time intervals less than or equal to the time interval T,its output remains at a substantially constant high level. Only if thetime interval between two successive received pulses is greater than theduration T of its output pulses, does the level of the output voltagefall to the rest level, where it remains until the receiptof thesucceeding input pulse. This is shown in waveform (d), FIG. 4.

In the arrangement of FIG. 1, the operating time T of one-shot 19 isdesigned to be greater than the normal period P between successivecharacter pulses delivered by one-shot 15, but less than twice thelength of period P. Therefore, the output of one-shot 19 is normally ata high level, termed the work level.

If the interval between two consecutive character pulses becomes greaterthan T, the output 20 of oneshot 19 falls to the rest level; i.e., anegative voltage pulse is generated. This negative pulse is applied tothe input of a one-shot 21, which has substantially the samecharacteristics as one-shot 15. Accordingly, a short positive pulse,waveform (e), is provided at output 22 of one-shot 21, in response tothe negative leading edge of the pulse received from one-shot 19.

This output pulse of one-shot 21 occurs once for each revolution of drum1 and disc 3 at a predetermined angular position of their rotation.Termed a turn pulse it is representative of the end of a revolution ofthe type-carrying drum. Accordingly, the turn pulse is applied tocircuit 17 to restore the initial condition of the binary counter.

Assuming, now, by way of example, that the set of the charactersrequired to be printed comprises the ten digits 0, l, 2 9. The surfaceof drum 1 is accordingly divided into eleven sectors of equal size, adifferent one of the 10 digits being located in each of 10 of thesesectors. The unused sector is that between the sector carrying the lastdigit of the set, 9, and the sector carrying the first digit 0. FIG. 3illustrates the developed surface of such a drum.

Corresponding to the exemplary drum organization of FIG. 3, timing disc3 is divided in eleven equal sectors, each such sector being providedwith a notch, except for the sector A, FIG. 2. Notches in adjacentsectors are equally spaced by a normal interval, whereas the intervalbetween the notches in the sectors adjacent to sector A is double thisnormal interval. Waveform (a), derived from the disc of FIG. 2,demonstrates a periodic irregularity 29, FIG. 4. This irregularitycorresponds to the absence of a pulse, as shown by the dashed line, andoccurs concurrently with the passage of notch-less sector A by readinghead 12.

Corresponding to irregularity 29, a square pulse is missing from theoutput waveform (b) of amplifier 13,

as shown by the dashed line 30. The corresponding pulse is missing inoutput waveform (c) of one-shot 15, as shown by dashed line 31.

Since the output signal of one-shot 19 is normally at work level, andfalls to the rest level only when a pulse is missing in the regularsuccession of received pulses, the absence of a pulse in waveform due tothe irregularity 29 in waveform (a), induces a rest pulse of duration2P-T in the output of one-shot 19, waveform (d).

In response to this rest pulse, a turn pulse 32 is provided at theoutput of one-shot 21, as shown by waveform (e). This turn pulserepresents the completion of a revolution.

In the example illustrated, the size of circular sector A has beenconsidered to be equal to the size of each of the remaining sectorswhich contains a notch. It is evident that the size of sector A can belarger or smaller. By increasing sector A, a greater operating safety isobtained at the expense of a lesser utilization of thecharacter-carrying surface of the drum if, as in the indicatedarrangement, the sector of the drum corresponding to the sector Acarries no print characters.

By reducing sector A, the utilization of the drum surface is increased,but a greater precision of the duration of operating time T of one-shot19 is required. In addition, a more precise regulation of the speed ofrotation of the drum is required to avoid the possibility of oneshot 19failing to generate the rest pulse.

The apparatus of FIG. 1 may be modified to provide full utilization ofthe surface of the type-carrying drum. FIG. represents such amodification of the apparatus of FIG. 1. In FIG. 5, the timing pulsesdelivered by oneshot are applied to one input of a two-input OR-gate 24and the output of one-shot 21 is applied to the other input of the ORgate. The output of OR-gate 24 is connected on input 25 of thecomparison and control circuit 17 in lieu of input 16 of FIG. 1.

On input 25, both turn pulses and character pulses are present. The turnpulse on input 25 may be used to control the printing of an additionalcharacter located in the sector of the drum which in the embodiment ofFIG. 1 had no character. Alternatively, the number of the sectors intowhich both the type-carrying member and the timing disc are divided maybe reduced by one, the number ofcharacters remaining unchanged.

Waveform (f) of FIG. 4 represents the sequence of pulses provided oninput 25 of FIG. 5. Turn pulse 32 is shown to be displaced relative tothe normal time of occurrence of the character pulses, the intervalbetween the preceding character pulse and the turn pulse being T+D,which is greater than P. Therefore, it is necessary to offset theposition of the character whose printing is controlled by the turnpulse, relative to the position of the other characters. FIG. 6 showsthe developed surface of a type-carrying wheel, or of a single ring of atype-carrying drum, suitable for use with the embodiment of FIG. 5.Character "O, which is located in the sector corresponding to the turnpulse, is offset by a distance x, corresponding to the time intervalDTP, from the center of this sector.

This embodiment of FIG. 5 might possible cause difficulties if theoffset character is too close to the following one. These difficultieswould not arise if the characters on the wheel or drum were separated bya relative- 1y large distance, although such a solution could beemployed only infrequently. These difficulties would be obviated if thesector allocated to the character corresponding to the turn pulse weresomewhat larger than the other sectors, thus providing for the offsetcharacter to be sufficiently distant from the following one.

According to anothervariation, shown in FIG. 7, delay means may beintroduced between the output of one-shot l5 and circuit 17. This delaymeans, comprising for example two one-shots 33 and 34, introduces adelay R for the character pulses, such delay R being determined so thatturn pulse 32 occurs exactly midway between the last and the firstcharacter pulses, as indicated by waveform (g) of FIG. 4. In thisinstance the angular position of the timing disc relative to thetype-carrying member must be suitably adjusted to accommodate thisadditional delay.

It is not essential to the operation of this invention that the drumrotate with a constant speed. The operating time of a one-shot depends,generally, on the charge or discharge time of a resistor-capacitorcircuit. Therefore, by employing variable-parameter components, such asvariable resistors or variable capacitors, in the one-shots, theoperating time of the oneshots involved, such as rapid-recovery one-shot19 and, in FIG. 7, one-shots 33 and 34, may be adjusted to accommodateto the speed of the drum. FIG. 8 illustrates such arrangement.

A tachometer 36 is coupled to motor 2 to provide an indication of theinstantaneous rotation speed, for instance by delivering a voltagesignal proportional to the speed. This voltage signal is applied to acontrol device 37, which operates to change the operating time of theone-shots involved; that is, in case of FIG. 1, of oneshot 19. Thus theduration T of the signal delivered by said one-shot 19 of FIG. 8maintains a constant ratio relative to the period P of thecharacterpulses, for all possible rotational speeds of the type-carrying member.

Having described herein a preferred embodiment of the invention, andsome variations of the same, with reference to specific application, itis apparent that other variations employing different and differentlyorganized circuit elements may be devised without departing from thescope and spirit of the invention.

In the instant where the set of characters is repeated more than onetime around the same ring, it is possible to identify the beginning orthe end of each set by leaving an angular sector devoid of anidentifying notch at the end of each set.

I claim:

1. In a device comprising movable members and means for identifying theposition of said members during the motion thereof, means coupled tomove with said movable members and provided with at least one orderedplurality of similar indicia regularly distributed, said pluralitycomprising a first indicium, a number of intermediate indicia, and alast indicium, each one of said intermediate indicia being located equaldistance from the preceding indicium and from the following indicium,said first and last indicia being separated by a substantially greaterinterval single transducer, means responsive to such indicia fordelivering signals representing said indicia, and means for detectingthe interval between the signals corresponding to said last and firstindicia, said interval being substantially larger than the intervalsbetween the signals corresponding to said intermediate indicia.

2. ln apparatus for signalling the presence of a reference point on acyclically movable member, said member having at least oneintelligence-bearing element in each of selected ones of a plurality ofcontiguous sectors which measure the entire extent of said member alongits direction of movement, the combination comprising: indicia-bearingmeans cyclically movable with said member, said indicia-bearing meanshaving an indicium disposed in each of said sectors but a sectorincluding said reference point, single transducer means disposed tosense said indicia along the path of their movement and to deliver anelementrepresenting signal each time an indicium passes said transducer,and means for receiving said elementrepresenting signals and responsiveto the intervals between consecutive ones of said signals for generatingan output signal when one of said intervals is greater than apredetermined duration, said predetermined duration being greater thanthe interval between consecutive element-representing signals deliveredin response to indicia in adjacent sectors, but less than the intervalbetween consecutive element-representing signals delivered in responseto indicia in sectors spaced by a sector including said reference point.

3. In apparatus for signalling the presence of a reference point on acyclically movable member, said member having a first plurality ofintelligence-bearing elements uniformly spaced apart along the extent ofsaid member in the direction of its movement, said member having asecond plurality of intelligence-bearing elements uniformly spaced apartalong said extent of said member, the spacing between said elements ofsaid first plurality being the same as the spacing between said elementsof said second plurality, the last element of said first plurality beingspaced apart from the first element of said second plurality along saidextent of said member by a distance greater than said uniform spacing,the combination comprising: indiciabearing means cyclically movable withsaid member, said indicia-bearing means having a first set of indiciauniformly spaced apart along the extent of said indiciabearing means inthe direction of its movement, said indicia-bearing means having asecond set of indicia uniformly spaced apart along said extent of saidindicia-bearing means, the spacing between said indicia of said firstset being the same as the spacing between said indicia of said secondset, the number of indicia of said first set equal to the number ofelements of said first plurality and the number of indicia of saidsecond set being equal to the number of elements of said secondplurality, the last indicium of said first set being spaced apart fromthe first indicium of said second set along said extent of saidindicia-bearing means by a distance greater than said uniform spacing ofsaid indicia, single transducer means disposed to sense said indiciaalong the path of their movement and to deliver an element-representingsignal each time that an indicium passes said transducer, and means forreceiving said element-representing signals and responsive to theintervals between consecutive ones of said signals for generating anoutput signal when one of said intervals is greater than a predeterminedduration, said redetermrned duration being greater than the mtervabetween consecutive element-representing signals delivered in responseto adjacent indicia of said first set, but less than the intervalbetween the element-representing signals delivered in response to saidlast indicium of said first set and the first indicium of said secondset,

4. The combination of claim 3 further including counting meansresponsive to each of said elementrepresenting signals to advance thecount state represented thereby.

5. The combination of claim 4 further including means responsive to saidoutput signal for resetting said counter.

6. The combination of claim 5 further including means for controllingthe reproduction of a symbol corresponding to a respective one of saidintelligencebearing elements each time one of said elementrepresentingsignals is generated.

7. The'combination of claim 3 wherein said member and saidindicia-bearing means revolve together at a substantially constantangular speed; wherein said spacing between said elements of said firstplurality, said elements of said second plurality, said indicia of saidfirst set, and said indicia of said second set is a single first angularvalue, and wherein said spacing between said last element of said firstplurality and said first element of said second plurality and betweensaid last indicium of said first set and said first indicium of saidsecond set is a single second angular value.

1. In a device comprising movable members and means for identifying theposition of said members during the motion thereof, means coupled tomove with said movable members and provided with at least one orderedplurality of similar indicia regularly distributed, said pluralitycomprising a first indicium, a number of intermediate indicia, and alast indicium, each one of said intermediate indicia being located equaldistance from the preceding indicium and from the following indicium,said first and last indicia being separated by a substantially greaterinterval single transducer, means responsive to such indicia fordelivering signals representing said indicia, and means for detectingthe interval between the signals corresponding to said last and firstindicia, said interval being substantially larger than the intervalsbetween the signals corresponding to said intermediate indicia.
 2. Inapparatus for signalling the presence of a reference point on acyclically movable member, said member having at least oneintelligence-bearing element in each of selected ones of a plurality ofcontiguous sectors which measure the entire extent of said member alongits direction of movement, the combination comprising: indicia-bearingmeans cyclically movable with said member, said indicia-bearing meanshaving an indicium disposed in each of said sectors but a sectorincluding said reference point, single transducer means disposed tosense said indicia along the path of their movement and to deliver anelement-representing signal each time an indicium passes saidtransducer, and means for receiving said element-representing signalsand responsive to the intervals between consecutive ones of said signalsfor generating an output signal when one of said intervals is greaterthan a predetermined duration, said predetermined duration being greaterthan the interval between consecutive element-representing signalsdelivered in response to indicia in adjacent sectors, but less than theinterval between consecutive element-representing signals delivered inresponse to indicia in sectors spaced by a sector including saidreference point.
 3. In apparatus for signalling the presence of areference point on a cyclically movable member, said member having afirst plurality of intelligence-bearing elements uniformly spaced apartalong the extent of said member in the direction of its movement, saidmember having a second plurality of intelligence-bearing elementsuniformly spaced apart along said extent of said member, the spacingbetween said elements of said first plurality being the same as thespacing between said elements of said second plurality, the last elementof said first plurality being spaced apart from the first element ofsaid second plurality along said extent of said member by a distancegreater than said uniform spacing, the combination comprising:indicia-bearing means cyclically movable with said member, saidindicia-bearing means having a first set of indicia uniformly spacedapart along the extent of said indicia-bearing means in the direction ofits movement, said indicia-bearing means having a second set of indiciauniformly spaced apart along said extent of said indicia-bearing means,the spacing between said indicia of said first set being the same as thespacing between said indicia of said second set, the number of indiciaof said first set equal to the number of elements of said firstplurality and the number of indicia of said second set being equal tothe number of elements of said second pluraliTy, the last indicium ofsaid first set being spaced apart from the first indicium of said secondset along said extent of said indicia-bearing means by a distancegreater than said uniform spacing of said indicia, single transducermeans disposed to sense said indicia along the path of their movementand to deliver an element-representing signal each time that an indiciumpasses said transducer, and means for receiving saidelement-representing signals and responsive to the intervals betweenconsecutive ones of said signals for generating an output signal whenone of said intervals is greater than a predetermined duration, saidpredetermined duration being greater than the interval betweenconsecutive element-representing signals delivered in response toadjacent indicia of said first set, but less than the interval betweenthe element-representing signals delivered in response to said lastindicium of said first set and the first indicium of said second set. 4.The combination of claim 3 further including counting means responsiveto each of said element-representing signals to advance the count staterepresented thereby.
 5. The combination of claim 4 further includingmeans responsive to said output signal for resetting said counter. 6.The combination of claim 5 further including means for controlling thereproduction of a symbol corresponding to a respective one of saidintelligence-bearing elements each time one of said element-representingsignals is generated.
 7. The combination of claim 3 wherein said memberand said indicia-bearing means revolve together at a substantiallyconstant angular speed; wherein said spacing between said elements ofsaid first plurality, said elements of said second plurality, saidindicia of said first set, and said indicia of said second set is asingle first angular value, and wherein said spacing between said lastelement of said first plurality and said first element of said secondplurality and between said last indicium of said first set and saidfirst indicium of said second set is a single second angular value.